The present invention relates to storage battery and relates, in particular, to a lead-acid storage battery for electric cars and the like that require particularly high output characteristics.
Prior art lead-acid storage batteries for electric vehicles have so far been used in electric golf carts, lawn mowers and the like that require an average current of (1/5) CA(5 hour rate current) and 1 CA or so even at the maximum. The discharge capacity of the foregoing batteries is limited by the characteristics of positive electrodes. In order to increase the discharge capacity of the positive electrodes (i.e., the utilization factor of active material), such efforts have been made as designing a grid that shows excellent current collecting characteristics, increasing the amount of active material used in positive electrodes and the like. On the other hand, in order to achieve a high degree of the battery""s weight efficiency, efforts have been made in designing lead-acid storage batteries to reduce the weight of the negative electrode as much as possible by such measures as reducing the usage amount of active material and increasing the mesh area of the grid.
In recent years, however, as the development of electric cars having a driving performance (acceleration and gradability) comparable to that of gasoline-driven cars is pursued actively, far more stringent high rate discharge characteristics are required of lead-acid storage batteries for electric cars than those for electric vehicles.
Although the low rate discharge capacity does not show much reduction during the lapse of charge/discharge cycles, the high rate discharge capacity is quickly degraded, thereby failing to meet the acceleration requirement and ending up with a shorter life than expected.
Therefore, it is important for a lead-acid storage battery intended for a cyclic operation with high rate discharge as encountered in electric cars and the like to prevent the degradation of high rate discharge capacity (i.e., enhanced life) during the lapse of charge/discharge cycles.
The foregoing degradation of high rate discharge capacity is mostly attributable to the degradation in negative electrode capacity. In order to prevent the above, such methods as reducing current density by the increased number of electrodes that is made possible through developing thinner electrodes, increasing the amount of active material used in negative electrodes and the like have been studied.
However, the prior art structure as described in the above causes lead sulfate to be accumulated in the active material of negative electrodes during the lapse of charge/discharge cycles, thereby creating a problem of reducing discharge capacity.
In addition, the increased amount of active material and increased number of electrodes have resulted in a drawback of reducing the battery""s weight efficiency.
A storage battery of the present invention includes a positive electrode, a negative electrode and an electrolyte material, in which the negative electrode has a first grid with a first grid configuration and a first active material provided on the foregoing first grid;
the positive electrode has a second grid with a second grid configuration and a second active material provided on the foregoing second grid; and
a first mesh area of the first grid is smaller than a second mesh area of the second grid.
Particularly desirable is that the first mesh area is about 50% or less of the second mesh area.
Further, particularly desirable is that the foregoing first and second grids are an expanded grid, respectively.
Accordingly, with the negative electrode, the contact area between the first active material and the first grid increases and further the average distance between the first active material and the first grid is reduced, thereby making the reaction of the negative electrode uniform and also improving the reaction of the negative electrode.
Furthermore, as a result of making the mesh area of positive electrode""s grid larger than the mesh area of negative electrode""s grid, the magnitude of battery""s capacity is governed by the positive electrode, thus allowing the utilization factor of negative electrode to be kept low in comparison with the negative electrode""s possible storage capacity.
In addition, the charge acceptability of negative electrode can also be enhanced. As a result, the prevention of an accumulation of lead sulfate is made possible, thereby improving high rate discharge cycle life.